As optical networks increasingly carry burgeoning Internet traffic, the need for advanced and efficient optical components rises. Optical communication systems permit the transmission of large quantities of information. Improved optical integrated circuits (OICs) are particularly needed. OICs come in many forms such as 1×N optical splitters, optical switches, wavelength division multiplexers (WDMs), demultiplexers, optical add/drop multiplexers (OADMs), and the like. Optical circuits allow branching, coupling, switching, separating, multiplexing and demultiplexing of optical signals without intermediate transformation between optical and electrical media.
Such optical circuits include planar lightwave circuits (PLCs) having optical waveguides on flat substrates, which can be used for routing optical signals from one of a number of input optical fibers to any one of a number of output optical fibers or optical circuitry. PLCs make it possible to achieve higher densities, greater production volume and more diverse functions than are available with fiber components through employment of manufacturing techniques typically associated with the semiconductor industry. For instance, PLCs contain optical paths known as waveguides formed on a silicon wafer substrate using lithographic processing, wherein the waveguides are made from transmissive media including lithium niobate (LiNbO3) or other inorganic crystals, silica, glass, thermo-optic polymers, electro-optic polymers, and semiconductors such as indium phosphide (InP), which have a higher index of refraction than the chip substrate or the outlying cladding layers in order to guide light along the optical path. By using advanced photolithographic and other processes, PLCs are fashioned to integrate multiple components and functionalities into a single optical chip.
One important application of PLCs and OICs generally involves wavelength-division multiplexing (WDM) including dense wavelength-division multiplexing (DWDM). DWDM allows optical signals of different wavelengths, each carrying separate information, to be transmitted via a single optical channel or fiber in an optical network. In order to provide advanced multiplexing and demultiplexing (e.g., DWDM) and other functions in such networks, arrayed-waveguide gratings (AWGs) have been developed in the form of PLCs.
A problem with OICs is polarization dependence of the waveguides, typically caused by waveguide birefringence. Waveguide birefringence is experienced in varying degrees with waveguides fabricated from the above-mentioned materials. For example, where the waveguides are formed by depositing a glass film on a silicon substrate, the difference in thermal expansion coefficient between the glass film and the silicon substrate base causes stress applied on the waveguides in a direction parallel to the surface to be different from that in a perpendicular direction.
Waveguide birefringence results, wherein the refractive index of the waveguides in the direction parallel to the substrate surface becomes different from that in the perpendicular direction. The birefringence, in turn, causes polarization dependence in the waveguides, where the optical path length difference (e.g., between adjacent waveguides) changes depending on the polarizing direction of light. In this situation, shifts occur between the transverse electric (TE) and transverse magnetic (TM) mode peaks, where the shift changes according to polarization. Consequently, the device characteristics change in accordance with the polarized state of the light provided to the device. For instance, the peak coupling in a particular channel or waveguide can vary according to the polarities of the various wavelength components, causing polarization dependent wavelength shift.
The polarization state in an optical network is affected by many parameters including stress on an optical fiber or waveguide. The varying loss associated with changing polarization is called polarization dependent loss. In other words, polarization dependent loss is a measure of component sensitivity to state of polarization. Polarization dependent loss is measured by monitoring the change in insertion loss as the polarization is changed through all of the possible polarization states with a polarization controller. Polarization dependent loss is expressed as the difference between the maximum and minimum insertion loss. The difference in insertion loss for one polarization compared to another is expressed in dB. Polarization dependent loss in optical network components affects a system's performance, especially when there are many components in the system.
OICs may contain one or more couplers. A coupler is a device for coupling optical power between two waveguides. Couplers made of planar waveguides consist of deposited films and other structures that typically have unmatched coefficients of thermal expansion. As a result, stress is present in the device. Stress in coupler waveguides in close proximity can be one or more of anisotropic and asymmetric. For example, the inner edges of coupler waveguides show higher stress compared to the outer edges. The stress is one or more of tensile stress and compressive stress.
The stress affects the refractive index of the waveguides through the stress optic coefficients and leads to unwanted rotations of the optical axes in the couplers. The rotation of optical axes facilitates the coupling between polarizations within the same waveguide as well as between different waveguides in close proximity. Such polarization coupling may result in increased polarization dependent loss within the device.
In OICs with a plurality of optical devices, such as a plurality Mach-Zehnder devices, polarization dependent loss bow is exhibited. Polarization dependent loss bow occurs when the end/edge channels of an array exhibit low polarization dependent loss, and polarization dependent loss increases as the middle/central channel is approached. This may be due to excessive etch loading on the end channels during their formation. Although it is generally desirable to reduce polarization dependent loss, an asymmetric occurrence of polarization dependent loss may be undesirable.
OICs contain metal lines, metal heaters, metal interconnects, and other metal films on top of the top cladding. These metal structures are required for the operation of the OICs. However, the metal structures on top of the top cladding often induce stress in the underlying waveguides. When the stress is asymmetric, birefringence is induced undesirably rotating the optical axes.